1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, to fabricating patterned magnetic media. More particularly, a lithographic process is used to form a servo pattern and a data pattern for a patterned magnetic media. Self-assembly structures are then built on the data pattern to further refine this pattern, but are not built on the servo pattern.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more magnetic recording heads (sometimes referred to as sliders) that include read elements and write elements. A suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to ride a particular height above the magnetic disk. The height depends on the shape of the ABS, disk spinning speed, pressure, and other variables. As the recording head rides on the air bearing, an actuator moves an actuator/suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
On a conventional disk, the magnetic surface of the disk is continuous. Binary information is recorded on the disk by polarizing a unit (called a bit) of the disk to be one polarity (1) or the opposite polarity (0). The smaller the bit, the more information can be stored in a given area. Present magnetic recording may achieve a unit as small as 18×80 nanometers. Each bit includes multiple magnetic grains, and the typical grain size is about 6 nanometers. Therefore, in a bit of size 18×80 nanometers, there are about 40 grains.
To increase the areal density of the magnetic disk, the bit size is reduced. If the grain sizes are kept the same for smaller bit sizes, then there would be a smaller number of grains in a bit resulting in smaller signal-to-noise ratio (SNR). If the grain sizes are reduced proportionally to keep the number of grains in a bit constant for smaller bit sizes, then the SNR would be the same. However, the super-paramagnetic effect may cause problems when grain sizes are reduced. The super-paramagnetic effect occurs when the magnetic grains on the disk become so tiny that ambient temperature can reverse their magnetic orientations. The result is that the bit is erased and the data is lost.
One solution to the problems posed by the super-paramagnetic effect is to pattern the magnetic disk. A patterned disk is created as an ordered array of highly uniform islands, with each island capable of storing an individual bit. Within each island, the magnetic materials are strongly coupled so that an island behaves as a single domain, in contrast to multiple domains in the continuous media. Because an individual magnetic domain is as large as an island, the patterned disk is thermally stable and higher densities may be achieved.
When data recording is performed on a magnetic disk, the read head and write head are positioned over the tracks based on a Positioning Error Signal (PES) that is read from servo regions on the disk. The servo regions include patterns that are used to guide the read and write elements to the proper position on the disk. The regions where the actual data is stored are referred to herein as the data regions.
There are problems encountered when fabricating patterned media. In data regions, the islands of the patterned media should be uniformly spaced with very tight distribution. The precise locations and sizes of the islands are important to the SNR and the Bit Error Rate (BER) of the data recording process. Also, to increase the areal density of the disk, the spacing and size of the islands have to be small which is challenging for the fabrication process as the requirements may be beyond the limits of the lithographic capabilities.
By contrast, the islands in the servo regions are typically larger in size than the data regions. Larger islands in a sync field of the servo region advantageously lead to larger magnetic amplitudes when read by a read element which can provide a more accurate determination of amplitude and timing in the positioning signal.
Another difference between the data regions and the servo regions is that the islands of the data region need to be uniformly spaced, whereas the islands in the servo regions are staggered with empty space in between. The arrangement of the servo region is as such to provide a sensitive PES. Servo regions may have complex patterns, may have open areas, and may tolerate the size and shape fluctuations of individual islands. The data regions on the other hand have a single regular pattern, and require highly uniform island arrangement in both the position and sizes.
One promising approach to improve the tolerance of the island locations and sizes is to grow self-assembly structures on top of the lithographically-defined template. Then the location and size tolerance will be improved to the level limited by the molecular mono-dispersity of the self-assembly molecules. Self-assembly structures are most stable on regular lattices, such as hexagonal close packed (HCP). A regular lattice is good for the data regions. However, in servo regions, the complex servo patterns do not necessarily conform easily to HCP or other simple lattices.
Patterned media is typically fabricated using nanoimprint lithography (NIL). Nanoimprint lithography is a high-throughput method for imprinting nanometer-scale patterns on a substrate. To imprint the nanometer-scale patterns on a substrate, a master template is first fabricated. The master template is not typically used for imprinting an actual substrate as it can be quickly worn out when a large number of imprints are needed. The master template is expensive and time consuming to fabricate, so the master template is rather used to fabricate a plurality of stamper tools. The stamper tools are then used for imprinting the substrates to fabricate the patterned media.
To fabricate a stamper tool, the master template is pressed into a layer of polymer stamper resist material to imprint the inverse pattern of the master template in the stamper resist material. Heat or ultraviolet (UV) irradiation may then be applied to the stamper resist material to harden the stamper resist material in the inverse pattern of the master template. The master template is then removed from the stamper resist material leaving a stamper tool having a desired pattern. The stamper tool may then be used to imprint a plurality of substrates that will form patterned media.
To imprint a substrate, the stamper tool is pressed against a thin layer of replica resist material deposited on the substrate to imprint the inverse pattern of the stamper tool in the replica resist material. The stamper tool is then removed from the replica resist material leaving a substrate with a desired resist pattern covering the substrate. An etching process, such as Reactive Ion Etching (RIE), may then be performed to pattern the substrate according to the resist pattern. A similar process is performed to pattern many substrates using the stamper tool.
The master template is thus fabricated to have a desired servo pattern and a desired data pattern so that these patterns may be transferred to a substrate to form a patterned magnetic media. It remains a problem to define the servo pattern and the data pattern on the master template, as these patterns do not conform to the same island size, shape, and distribution.